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Sequence of Operation
Cooling
A – First Stage Low Capacity
Transformer from indoor unit supplies 24VAC power to
the thermostat and outdoor unit controls.
1− Internal wiring energizes terminal O by cooling mode
selection, energizing the reversing valve. Cooling de-
mand initiates at Y1 in the thermostat.
2 − The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3− K1−1 N.O. closes energizing compressor B1 and out-
door fan motor B4.
4− Solenoid L34 is NOT energized. The slider ring re-
mains open limiting compressor to low capacity.
B – Second Stage High Capacity
5− Second stage thermostat demand goes through Y2 on
the defrost board and energizes rectifier plug D4. D4
converts the AC voltage to DC voltage and energizes
L34 unloader solenoid. L34 then closes the slider ring,
allowing the compressor to operate at high capacity.
Heating
A – Low Capacity
1− Internal wiring de−energizes terminal O by heating
mode selection, de−energizing the reversing valve.
Heating demand initiates at Y1 in the thermostat.
2 − The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3− K1−1 closes, energizing the compressor and outdoor
fan motor.
4− Solenoid L34 is NOT energized. The slider ring re-
mains open limiting compressor to low capacity.
B – High Capacity (Ambient temperature above
defrost board Y2 lock−in temperature)
1− Room thermostat in heating mode. Room thermostat
outputs Y1 and Y2 (if applicable to that room thermo-
stat) signal to the defrost board in the heat pump and to
the indoor unit.
2− The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3 − The defrost board sends 24 volts through Y2 OUT to
the L34 compressor solenoid plug. The 2− wire com-
pressor solenoid plug converts the 24volt AC outputs to
a 24volt DC signal input to the L34 internal high capac-
ity solenoid valve in the compressor.
4 − K1−1 closes, energizing the compressor and outdoor
fan motor through the normally closed fan relay con-
tacts on the defrost board. The compressor runs high
capacity.
B – High Capacity (Ambient temperature below
defrost board Y2 lock−in temperature)
1 − Room thermostat in heating mode. Room thermostat
outputs Y1 signal to the defrost board in the heat pump
and to the indoor unit.
2 − The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3 − The defrost board
Y2 locks in
sends 24 volts through
Y2 OUT to the L34 compressor solenoid plug. The plug
converts the 24volt AC outputs to a 24volt DC signal in-
put to the L34 internal high capacity solenoid valve in
the compressor.
4 − K1−1 closes, energizing the compressor and outdoor
fan motor through the normally closed fan relay con-
tacts on the defrost board. The compressor runs on
high capacity.
Defrost Mode
When a defrost cycle is initiated, the control energizes the
reversing valve solenoid and turns off the condenser fan.
The control will also put 24VAC on the W1" (auxiliary heat)
line. The unit will stay in this mode until either the coil sensor
temperature is above the selected termination tempera-
ture, the defrost time of 14 minutes has been completed, or
the room thermostat demand cycle has been satisfied. (If
the temperature select shunt is not installed, the default ter-
mination temperature will be 90°F.) If the room thermostat
demand cycle terminates the cycle, the defrost cycle will be
held until the next room thermostat demand cycle. If the coil
sensor temperature is still below the selected termination
temperature, the control will continue the defrost cycle until
the cycle is terminated in one of the methods mentioned
above. If a defrost is terminated by time and the coil temper-
ature did not remain above 35°F (2°C) for 4 minutes the
control will go to the 34−minute Time/Temperature mode.
